1. Field of the Invention
The present invention relates to a zoom lens which is suitable for a 35 mm single-lens reflex camera and, more particularly, to a zoom lens suitable for an autofocus camera in which aberrations are satisfactorily corrected although the zoom lens has a focal length at the wide angle end of about 70 mm, a zoom ratio of about 3:1, and a relatively large aperture of an F number of about 2.8.
2. Description of the Related Art
In recent years, in an autofocus camera which is increasingly the subject of development, not only is the performance of accurate focusing important, but also the attainment of rapid focus speed. The focus speed is determined mainly by power supplied to a focus motor, the efficiency of the power focus motor, the load of a lens barrel, the weight and extension of the focus lens, the computation processing speed of the autofocus, and the like. Of these factors, the factors which influence focus speed the most are the weight of the focus lens and the extension thereof. It is therefore important to achieve an optical design in which the influence of the weight and extension factors is limited to be small.
A zoom lens, having a focal length at the wide angle end of about 70 mm, a zoom ratio of about 3:1, and an F number of about 2.8 (constant during zooming), is generally formed of a first lens group having a positive refracting power, a second lens group having a negative refracting power, a third lens group having a positive refracting power, and a fourth lens group having a positive refracting power. Such a lens is well known and afocal zooming is performed by moving the second and third groups during zooming while the first and fourth groups are fixed and focusing is performed by the first lens group (front lens focusing) or a part of the first lens group (front lens inner focusing). An example of front lens focusing is proposed in, for example, Japanese Patent Laid-Open Nos. 62-108218, 64-39542, and 63-58324. Although the front lens has a relatively small aberration, the weight of the front lens is fairly heavy when it has a large aperture, for example, when the F number is F2.8. When the closest focusing distance is set at a relatively short distance, for example, about 1.5 m, the extension amount is increased relatively. Therefore, it cannot be said that the zoom lens is suitable for an autofocus camera. When the power of the front lens is increased in order to decrease the extension amount, the spherical aberration, in particular, at a near distance on the long focal point side, will become great and be undercorrected, which is problematic.
An example of front lens inner focusing is disclosed in, for example, Japanese Examined Patent No. 61-053696. Since focusing is performed by a lens group disposed relatively on the image side of the front lens, it is possible to decrease the lens diameter to some extent, and as a result, lessen the weight of the focus lens to some extent. There are advantages to this type of system even if the refracting power is increased to decrease the extension amount. For example, little aberration will occur due to the floating effect with the lens group of the front lens on the object side. Despite the advantages, such a system is nonetheless not suitable for an autofocus camera in terms of its weight.
Further examples in which the weight of the focus lens is reduced are disclosed in, for example, Japanese Examined Patent Nos. 1-25044 and 1-25043. These examples are designed to focus using a part of the fixed fourth lens group (relay); however, there are problems associated with this system, for example, aberrations, such as a spherical aberration or an astigmatism, will be large though the weight of the focus lens is fairly small.